Thanks for reaching out to the Community! I would be happy to assist you!
The default welcome email on the accounts is an automated email that will be sent to all new sign-ups by the system. Once you have it customized and activated, you don't have to do anything else! That email will ONLY go to new contacts that have come in through a sign-up form.
If you would like to create a regular email campaign to manually send out to your contacts, you can do that also.
The other option is to use an automated email series to send it out automatically to contacts you add to a certain list. That option is only available in the Email Plus plan, but would allow you to send it to contracts other than the ones coming in through a sign-up form, such as new contacts you add yourself.
Please let us know if there is anything else we can do for you!